The polyurethane prepolymers, which are obtained by reacting an excess of tolylene diisocyanate with a polymeric diol, are commonly employed to prepare cast polyurethane elastomers when cured or chain-extended with organic aromatic diamines. The more commonly employed diamine is 4,4'-methylenebis(o-chloroaniline) which, when employed to cure prepolymers prepared from tolylene isocyanate, provides elastomers having excellent physical properties. However, the possible implication of this diamine as being carcinogenic has made it desirable to find a substitute which imparts equal or better physical properties and which is inexpensive and readily available.
The present invention represents the culmination of a long series of investigations undertaken to find an inexpensive, readily available substitute.
It is well recognized in the art that 2,4-tolylene diisocyanate contains 2 unequally reactive isocyanate groups and that prepolymers prepared from a major proportion of this isomer have terminally hindered isocyanate groups. These prepolymers are generally chain-extended with diamines which have 2 equally reactive amino groups, since it is important that the rate of the chain-extending reaction be reasonably fast. A diamine having 2 unequally reactive amino groups has a generally slower rate of reaction. Therefore, 4,4'-methylenebis(o-chloroaniline) has been the preferred curing agent.
Accordingly, it is a primary object of this invention to provide an inexpensive, readily available substitute for 4,4'-methylenebis(o-chloroaniline).
Another object of the invention is to provide a new and improved diamine curative system for polyurethane prepolymers.
It is still another object of this invention to provide a polyurethane elastomer with improved low temperature properties, that is having a low glass transition temperature.
Lastly, it is an object of the invention to provide an improved process for chain-extending polyurethane prepolymers haveing terminal isocyanate groups.
Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by the practice of the invention, the objects and advantages being realized and attained by means of compositions and improvements, particularly pointed out in the appended claims.